There are YT videos that cover the process, but I don't have any specific links. Just thought I point it out as a resource.

However, a few things I can tell you are:

1) Don't try and use it straight out of the container. It is water based and it needs to be thinned with a just a tiny bit of water (in a separate container). You want it thinned to a consistency of gyprock joint compound (another analogy is toothpaste, I don't like that description). Basically not too thick, and not too soupy.

2) When you spread it over the timber, use an applicator like a plastic 75mm putty knife or even an old credit card, and spread it across the grain.

3) Don't try to do it all in one go. It will take at least 3 applications (especially with ash). You might get away with 2, but I reckon 3 minimum. Also, if you put it on too thick, it takes forever to sand back. That stuff is HARDER than the wood when it's dry.

4) Start sanding with P120 paper (aluminium oxide/non-clog) but just use that to get the initial bulk off the "top", then switch to P240.
If you go too long with the 120, it becomes counter-productive because the larger aggregate (courser) on the paper can get into the grain lines and scrub out the filler you just put in!

FWIW, doing a good, thorough job takes time so expect it to be a 30 minute one-and-done job. It can actually be a PITA, but totally worth the effort.